HONESTY might be the best policy, but there's a point where it becomes all too hard.

That's what researchers from Harvard University in America believe, after a series of experiments proved humans are more likely to lie over the course of the day.

Their study concluded that men and women find it easier to be deceptive in the afternoon than in the morning, the Daily Mail reports.

They believe we lose the self-control to be honest as we get tired, so we find it easier to trick and lie.

Ironically, the more fundamentally honest a person was, the more susceptible to the phenomenon they were.

The US researchers were conducting experiments on what's called the morning morality effect.

In one of the experiments, a group of young men and women were shown a pattern of dots on a computer and asked to say whether there were more on the right or left of the screen.

In some cases it was hard to tell but in others the correct answer was clear.

The volunteers were given cash for every round they played. But the payments weren't for getting the answer correct - instead participants were given ten times as much cash for choosing the right side of the screen rather than the left.

This encouraged fibbing and those who played the game in the afternoon were more likely to say the dots were on the right when they were clearly on the left.

A nose this big would likely be achieved in the afternoon.Source:Supplied

A second experiment tested whether people are more aware of ethics in the morning than in the afternoon.

Men and women were given words with missing letters and asked to complete them.

Those taking part in the morning were more likely to see E_ _ _ C _ _ and _ _ RAL and think of the words 'ethical' and 'moral'.

Those doing the test in the afternoon tended to form the words 'effects' and 'coral'.

"We found that people are vulnerable to a gradual depletion of self-regulatory processes as a result of unremarkable daily activities," the researchers concluded.

"This depletion can, in turn, lead them to act in ethically questionable ways.

"Unfortunately, it might be that the most honest people are most susceptible to the negative consequences associated with the morning mortality effect.

"In other words, our findings suggest that the mere time of day can lead to a systematic failure of good people to act morally."